Investigations with cats and monkeys have shown that binocular anomalies present during a susceptible period of development may result in loss of binocularity, including stereopsis, and the development of amblyopia. Clinical observations indicate a similar susceptibiltiy in humans. The extent to which the visual system can recover from these deficits depends, in part, on the time and adequacy of correction during the susceptible period. The implication of this work is theoretically straightforward. If the patient can be treated at the proper time during the critical period of development, his prognosis for the development of normal visual functioning may be made more favorable. Although susceptible periods for development of the human system are recognized, their courses are nor clearly delineated. The major objective of the proposed research will be to ascertain the timing, and variations in the sensitivity, of periods of susceptibility of the human visual system to modification by occlusion and misaligned visual axes. We shall try to define the relationship between the loss of binocularity and the development of amblyopia. The main method will entail taking sequential measures of resolution, both grating and stereoacuity, before, during, and after therapeutic intervention. In addition, measures will be made of the development of the relative gain between nasotemporal and temporonasal motion-induced optokinetic nystagmus. The outcome should be relevant to determining the optimal time for surgical intervention and for strabismic therapy. The proposed research is a natural extension of ongoing work in our laboratory which deals with the development of vision in normal and astigmatic infants.